Several in vitro studies have shown that cyclin A gene alteration in the cell cycle plays an important role in carcinogenesis. We immunohistochemically examined the expression of cyclin A protein in 120 patients with transitional cell carcinoma (TCC) of the renal pelvis and ureter, including adjacent dysplastic lesions to determine their significance for the tumor behavior and patient prognosis. Cyclin A immunostaining of the nucleus was observed in 29 tumors (24.2%). Furthermore, 17 cyclin A-positive tumors (58.6%) had dysplastic lesions positive for cyclin A antibody. The prevalence of cases exhibiting cyclin A staining was higher in the high grade (P < 0.01) and invasive tumors (P < 0.05) than in the other types of tumors. In the selected 117 cases, patients whose TCCs expressed a high level of cyclin A protein had a significantly poorer prognosis than those without cyclin A expression (P < 0.01). These in vivo findings provide the first evidence for frequent and redundant cyclin A protein overexpression in TCC and suggest that cyclin A overexpression is related to the tumor behavior and patient prognosis. In addition, our observations indicate that overexpression of cyclin A may be one of the early events, at least in some cases, in the carcinogenesis of TCC.